In my prior U.S. Pat. Nos. 4,020,708; 4,089,228 and 4,569,240, there are disclosed various forms of motion generating mechanisms in which a constant rotary speed input from a prime mover is transformed into a cyclic output in which, during each cycle, a rotary output member is accelerated during the first portion of the cycle and decelerated through the final portion of the cycle. The rotary output member may, for example, be a gear which is meshed with a rack to drive the rack in a linear stroke cycle from a start to a finish position in motion such that a plot of the linear velocity of the rack versus its distance from its start position would resemble a cycloidal curve.
In the mechanism disclosed in my U.S. Pat. No. 4,569,240, a rotary input shaft drives a planet gear in rotation about a sun gear which is fixed. The planet gear carries a crank pin offset from its axis of rotation which is received within a radial slot in a relatively large diameter gear mounted for rotation about an axis coaxial with that of the fixed sun gear. This relatively large gear is in turn meshed with the input gear of a motion multiplying gear box whose output in turn is coupled to a rack or other driven member. In this arrangement, the planetary gear-crank mechanism transforms the constant speed rotary input into a cyclically variable speed rotation of the relatively large diameter gear assembly. The system is so designed that the relatively large gear assembly will rotate through one complete revolution for each working cycle or stroke of the driven output member. The multiplying gear box driven by the relatively large diameter gear assembly converts this single revolution of the relatively large gear assembly into the necessary number of revolutions to drive the output member the distance required to perform its intended work function.
In many applications, it is essential that the output member driven by the motion generating mechanism be located precisely at the same fixed reference point at the end of each working cycle. The precision with which this may be accomplished is largely dependent upon the amount of backlash which exists between the relatively large diameter gear assembly and the meshed input gear of the multiplying gear box. Any backlash which exists between these two gears is multiplied by the gear box.
The relatively large diameter gear assembly, for reasons described below, is manufactured with a relatively large central opening. In a typical application, this gear may have a pitch circle diameter of 16 inches. Because of its configuration, its size, and hardening processes, it is extremely difficult to manufacture this particular gear to precise tolerances. Typically, measurements of the finished gear will reveal that its pitch circle is not precisely circular. Experience has shown that this deviation is unpredictable and in most cases, for all practical purposes, uncorrectible. This has resulted in an unacceptably high scrap rate. Any substantial deviation from true circularity is directly reflected in the backlash characteristics of such a gear. The deviation from true circularity may be such that when meshed with another gear, backlash may be unacceptably high (loose) when one point of the gear is in mesh with the mating gear and may be negative--that is binding--at another point.
The present invention is directed to minimizing the effect of the problem described above in the assembled motion generating mechanism.